1. Field of the Invention
The invention relates to a method for processing an X-ray image. The invention also relates to an X-ray image intensifier comprising an entrance screen with a photocathode. The invention further relates to an X-ray examination apparatus comprising an X-ray detector for deriving a low-energy radiation image from an X-ray image and for deriving a primary image signal from the low-energy radiation image and a correction system for deriving a corrected image signal from the calibration signal and the primary image signal.
2. Description of Related Art
Such a method and such an X-ray image intensifier and X-ray examination apparatus are known from the German Offenlegungsschrift DE 40 01 010.
The known method requires the formation of a separate test X-ray image of a test plate made of a homogeneous material and having a uniform thickness. From the test X-ray image a test video signal is derived by means of a video camera and the test video signal is stored in a memory unit. When a patient is to be radiologically examined an X-ray image of the patient is formed and converted into a video signal. The X-ray image is processed in that the video signal of the X-ray image of the patient is divided by the test video signal. Thus, the signal levels of the test video signal represent correction numbers which are used to correct the primary image signal formed by the video signal of the X-ray image.
Accurate correction of the video signal would be achieved only if the circumstances during the X-ray imaging of the test plate were identical to the circumstances during the X-ray imaging of the patient. However, in practice numerous aspects of the X-ray imaging circumstances will change between the formation of the test X-ray image and the X-ray imaging of the patient. Consequently, the test video signal often quite inaccurately represents perturbations of the primary image signal. Notably, because as a special test plate is imaged to form the test X-ray image, the known method does not enable the test X-ray image to be obtained when the patient is positioned for irradiation with the X-ray beam. On the contrary, often a substantial period of time will elapse between the forming of the test X-ray image and the X-ray imaging of the patient. During that period the electrical voltages applied to an electro-optical system of an X-ray image intensifier may change, either intentionally when the magnification of the X-ray image intensifier is changed, or unintentionally due to drift of the electrical voltages and/or there may be a change in the orientation of the X-ray image intensifier with respect to an external magnetic field such as the terrestrial magnetic field. Moreover, during said period temperature changes may take place. Therefore, the previously formed test X-ray image is often inadequate for accurate correction.